Generally, natural gas exits gas wells at up to several hundred bar of pressure. This is regulated to be suitable for transportation in gas networks. This pressure is maintained with the aid of compressors. Therefore, the transportation system has a significant potential energy which is lost as the pressure is successively reduced within the distribution system so ensure that customers receive a suitable pressure.
Most countries use a similar system of transportation of gas across the territory, followed by regional distribution. In the UK, there are three broad groups of pressure reducing steps that take the pressure from 60-80 bar down to household mbar pressure. It is estimated that within the UK grid, there is up to 1 GW of accessible mechanical energy from gas expansion. This energy is mainly lost at Pressure Reducing Stations and there is a growing argument that the energy could and should be recovered.
It is therefore desirable to capture the mechanical (kinetic) energy that results as the gas depressurises and loses potential energy.
The incumbent pressure reduction technology involves the use of a simple orifice to reduce the pressure. This can be in the form of a modulating valve and a control system, which is commonly called a Joule Thompson valve. In gas networks, there are different names for pressure reduction. In the UK, the higher pressure systems are called pressure reduction stations (PRS), or a transmission regulator station (TRS). In the United States, the stations are called pressure letdown stations (PLS).
As the gas travels through the Joule Thompson pressure reduction valves, there is a temperature drop associated with the isenthalpic adiabatic expansion of the gas. If uncontrolled, the cold pressurised gas would allow condensation and freezing of hydrates, which can result in damaged equipment and blocked pipes. As shown in FIG. 1, this cooling presently necessitates the use of a preheating step.
Typically, the preheating technology is relatively rudimentary. It is a combustion technology, which heats a fluid in a reservoir. The gas travels through a heat exchanger in the reservoir to collect the energy. The required pre-heat depends on the initial pressure, pressure change and gas composition. For example, for typical UK natural gas composition and an input pressure of 30 bar, the temperature will fall by approximately 0.6° C. for every bar decrease in pressure. Thus, for a downstream pressure of 5 bar, a 15° C. temperature drop will be produced, which if uncontrolled will cause the output temperature to be −5° C. The amount of energy required to heat the gas is relatively small compared with the amount of chemical energy travelling in the pipe flow. However, with over 14,000 PRS within the UK, this represents a significant consumption of gas and release of CO2, which is undesired.
Turbo expanders have been used for many years to recover energy from expanding gas flows. They come in a variety of sizes and efficiencies. Most use a high speed turbine, with a less common method using a positive displacement system similar to a screw-expander arrangement. By function, both have similar efficiencies (typically 30-85% isentropic efficiency) and effects on temperature and pressure. All designs can be coupled with an electrical generator to convert the expanding gas into electrical energy. The electrical power taken from the generator is then typically conditioned for use or export. By nature, turbo expanders encourage isentropic adiabatic expansion which reduces the temperature of the gas by typically 5 times more than Joule Thompson pressure reduction valves.
There appears to be several reasons why turbo expander or screw-expander technology has not been widely implemented on the gas networks:                The electrical energy generated by a turbo expander is approximately 85% of the pre-heating requirement (although it is noted that the electricity generated is worth more per MWhr than the consumed gas).        The increased gas consumption required for pre-heat results in an increase in the carbon footprint of the distribution system, at a time when there is considerable encouragement to decrease the carbon footprint.        To allow the electrical energy to be exported at low cost, the PRS needs to be in close proximity to an electrical substation with sufficient generation capacity to accept the electrical input from the turbo-expander generator, significantly reducing the number of available sites.        